1. Field of the Invention
The present invention relates to beam scanning antennas used for radar antennas, mobile communication antennas, satellite broadcasting receiving antennas, satellite-installed-antennas, and so forth.
2. Description of the Related Art
In antennas used for radars, mobile communications, satellite broadcasting reception, and so forth, a function which freely scans a beam in a desired direction is important. For example, when a mobile communication is performed, a mobile station (for example, an automobile or an airplane) is usually moving. Thus, a beam radiated from the antenna of the mobile station is not always directed to a predetermined direction. In this case, the beam of the antenna should be tracked to the predetermined direction (namely, the direction of an objective station which receives and/or transmits radio waves). As a means for scanning the beam of an antenna, the following methods have been employed.
As a first method, a beam is scanned by a mechanical driving means. FIG. 103 shows the construction of an example of a mechanical driving means which scans a beam. The mechanical driving means for scanning the beam requires an elevation (vertical plane) driving member and an azimuth (horizontal plane) driving member. In the mechanical driving means, the beam scanning process does not adversely affect the electric characteristics (such as radiation directivity) of an antenna 1.
However, the construction of the driving mechanism becomes complicated. In addition, there is a problem on durability of the rotary joint used for the driving mechanism. In particular, in a mobile communication, a lower and flat antenna is required. Thus, the mechanical driving method is disadvantageous.
As a second method, an electrical beam-scanning method is known. FIG. 104 shows the construction of an example of an electrical beam-scanning method. As shown in the figure, a plurality of antenna elements 2 are electrically connected to corresponding phase shifters 3. In the receiving operation, signals received from the antenna elements are combined by a feeding circuit 4. In the transmitting operation, a signal is distributed to the antenna elements 2 by the feeding circuit 4. The exciting phases of the antenna elements are adjusted by the phase shifters so as to scan the beam in a desired direction. In this method, since all the antenna elements 2, the phase shifters 3, and the feeding circuit 4 are disposed on the same plane, the beam scanning antenna can be thinly formed.
However, in this method, since each antenna element requires a phase shifter, the size of the beam scanning antenna becomes large. In addition, the cost of the antenna increases. Moreover, to control the phases of the phase shifters 3, a control circuit is required. Thus, the construction of the antenna becomes complicated.